The book that has most influenced me...
I had read a chapter by Michael Goulder on the synoptic problem in an edited volume on alternative approaches to the New Testament. The title was something to do with houses built on sand, I think. The gist of his argument was that the elasticity of the two source hypothesis was its major flaw, and he used the work of two philosophers of science, Karl Popper and Thomas Kuhn, to show this. This led to me reading A.F. Chalmers' book, What is this thing called science? (I remember the cover has a cat on it which was apparently significant for a previous edition but had now been removed from the text and remained only on the cover - weird what you remember, eh). The book was itself heavily influenced by the work of Paul Feyerabend and his Anarchistic Theory of Knowledge, but what I remember most was Chalmer's rehearsal of the problems of induction, of Popper's falsificationism, and his critical comments on Kuhn's paradigms and Lakatos' research programmes.
Though this book was the start of my journey into the work of the pragmatists, the development of my thinking was never just theory led. The simple fact of the matter was that the methodological and exegetical questions that arose at each level of my studies always seemed to be best answered by the work of people like Stanley Fish. That this still seems to be the case is why I count this book as the one that has marked me the most.